I. Field of the Invention
The design and manufacture of an ion source is provided with a rapid beam current controller for experimental and medicinal purposes. More particularly, the design and manufacture of a laser ion source is provided with a magnetic field applied to confine a plasma flux caused by a laser ablation.
II. Background of the Related Art
High-energy ionizing radiation has continuously been used in high energy physics for the last half a century. More recently, however, high-energy ionizing radiation has shown promising results in the medical field and, in particular, in the treatment of cancerous tumors because hadronic matter, i.e., protons and light ions (e.g., carbon), have the advantage of easily penetrating the body and then depositing their energy at a depth immediately before the particles/ions come to rest determined by “Bragg peak.” The light ions also have shown an increased relative biological effectiveness in treating cancerous tumors. Due to these advantages as compared to conventional radiotherapy, hadron therapy facilities have been build with increased frequency.
The use of different hadron beams needs the availability of powerful ion sources which are time-stable and provide high-quality beams of different light ions. At present, electron cyclotron resonance (ECR) ion sources mostly provide the particle beams for hadron therapy. The use of ECR ion sources is based on resonantly coupling microwave power to a plasma by matching the microwave frequency to the electron cyclotron frequency in the magnetic field where the plasma is confined.
Laser ion sources (LIS) have been proposed as an alternative ion source because the LIS has two major advantages over other types of ion sources. The first feature is a high plasma density. The LIS creates plasma from dense solid material, while other types of ion sources normally start from gas. A single laser shot from a conventional tabletop laser can generate a large number of ions. For example, a 2 J Nd—YAG laser shot generates about 2×1014 ions from an aluminum target. The second advantage is that the laser-produced plasma has an initial expanding velocity normal to the target. The laser-generated ions can be transported in a neutralized plasma state.
To utilize these two advantages, Okamura et al. (17th Inter. Symp. on Heavy Ion Inertial Fusion, 2008; incorporated herein by reference in its entirety) proposed a method of combining a laser ion production and injection for use in a low charge state heavy ion production configured as a direct plasma injection scheme or (DPIS). FIG. 1A shows a DPIS scheme of Okamura et al. In this scheme, a solid target 11 is placed in an electrically isolated enclosure 17 biased to HV power supply which is in a vacuum chamber 10. The vacuum chamber 10 is directly connected to a radio frequency quadrupole (RFQ) linear accelerator 30 via a plasma drift section 20. A high power laser 40, i.e., generally between 108 to 1013 W/cm2, is focused onto the solid-state target 11 through windows 13 via an optical assembly 12 (including a plurality of flat surface mirrors and a convex lens) to produce a dense plasma, which contains highly charged ions. A laser produced plasma adiabatically expands in the direction 15, which is perpendicular to a target surface, as shown in FIG. 1B. Simultaneously, the plasma expands three dimensionally with a large momentum spread. This expansion makes a plasma pulse width longer and a current density smaller. The induced plasma is then pushed out from the enclosure 10 and the plasma drift section 20 into the RFQ cavity 30 through the extraction point 31. Inside the RFQ cavity 30, the ions from the neutral laser plasma are extracted by the electric field and are immediately captured by the RF quadrupole focusing force of the RFQ electrodes 33 within the RFQ. As a result, the high density ion beam is efficiently accelerated through the RFQ.
While laser ion sources are very powerful and can provide low charge state, low emittance and high ion yield, they still suffer from numerous drawbacks especially if a high charge state is desired. For example, although the peak current is high, the pulse width of the beam is too short for some applications such as the acceleration of ions in the synchrotron and it is difficult to change the beam current within a short time frame, a prerequisite for successful hadron therapy. Also, while, the plasma pulse width at the entrance of the RFQ can be extended to increase ion beam pulse width, primarily regulated by extending plasma drift distance, unfortunately, the injected current to the RFQ becomes too small and unworkable.
Therefore, it would be desirable to have a laser ion source (LIS) that overcomes the shortcomings of the prior art including the difficulty of (1) changing the ion beam current on pulse to pulse basis, (2) controlling the ion pulse duration and shape, and (3) independently changing the ion pulse length and the beam current.